Hypothermia, which may be defined as body temperature below 36 degrees Celsius occurs for a variety of reasons during surgery. The ability of a patient to conserve heat and vary heat production during surgery and post operatively is disturbed by both anesthesia and exposure of the patient to the operating room environment. This loss of body heat occurs by conduction, evaporation, convection and radiation. Metabolic heat production by an anesthetized adult patient approximates 60-70K calories per hour. Conduction to the operating table and surgical drapes account for less than 10% of heat loss. Evaporation loss occurs from exhaled air and from exposed body cavities, and amounts to roughly 25K calories per hour. Variations in anesthesia circuitry including low flow systems, closed circuits, to and from systems and heat humidifiers can reduce evaporation losses to 10-15 calories per hour. Convection losses are naturally dependant upon ambient temperature. At 21 degrees Celsius the patient loses approximately 80 calories per hour which in itself is greater than the amount of heat the patient can produce over a like period. In addition, radiant heat loss is approximately 100 calories per hour.
Additional causes of hypothermia include cold exposure, major trauma, massive blood transfusions, sepsis in elderly patients, prolonged periods of unresponsiveness, maternal hypothermia and neonatal hypothermia in low birth weight infants.
Known methods used to reduce heat loss during and after a surgical procedure include heated and moisturized inhalation anesthetics, radiant heaters, warming blankets, reflecting blankets, increasing the temperature of intravenous infusions and blood transfusions and warm irrigation. It is also possible to increase ambient temperature, but this may cause operating personnel discomfort.